Chinese authorities have started prosecuting 12 Hong Kong activists caught while allegedly fleeing the city by boat for Taiwan, officials said Wednesday.

The group was caught by the coastguard some 70 kilometres (43 miles) southeast of the city on August 23, authorities said previously, before being handed to police in the mainland metropolis of Shenzhen.

In a social media notice, the People's Procuratorate of Yantian District in Shenzhen said Tang Kai-yin and Quinn Moon were suspected of organising for others to cross the border.

Eight others stand accused of an illegal border crossing.

Two minors face non-public hearings, according to the notice.

The activists -- whose arrests were formally approved in September -- had disappeared into China's opaque judicial system since their capture, with lawyers struggling for access and family members expressing fear over their fate.

Some of those aboard the boat faced prosecution in Hong Kong for activities linked to last year's huge and often violent pro-democracy protests.

Lu Siwei, a mainland Chinese lawyer working on the case, told AFP the court was expected to hand down a verdict on the case within two to three months.

Although Hong Kong has its own internationally respected legal system -- where detainees are promptly produced after their arrest and tried in open court -- the mainland's judicial system is notoriously opaque, with conviction all but guaranteed.

In June, Beijing imposed a new security law on Hong Kong, announcing it would have jurisdiction for some crimes and that mainland security agents could openly operate in the city.

With Beijing clamping down on Hong Kong's democracy movement, Taiwan emerged as a sanctuary, quietly turning a blind eye to residents turning up without proper visas or paperwork.

© AFP